This invention relates to a power wrench of the type having a housing with a pneumatic rotation motor drivingly connected to an output shaft, a pressure air inlet passage and an exhaust air outlet passage in the housing, a manually operable throttle valve located in the air inlet passage, and an adjustable exhaust air flow restricting valve in the outlet passage.
In prior art, manually adjustable exhaust flow restricting valves have been used for reducing the idle or low-load speed of power wrench motors, thereby reducing the risk for overtighten stiff screw joints and for avoiding premature motor shut-off at such joints. Particularly at the impulse type of tools there is a problem with a premature shut-off, because when tightening a stiff joint the very first impulse generated will be of a very high magnitude as a result of a high running down speed. There is also a risk that the target torque level is reached and even passed by such an initial high energy impulse.
Previously known exhaust flow restriction valves at power wrenches are of two main types, namely I)the manually adjustable type, which is intended to be selectively set in a number of alternative positions with varying flow restricting action, and II)the self-adjusting type controlled by the actual exhaust flow acting on a valve element in the opening direction against a spring biassing the valve element in the closing direction.
Both of these two previously known types of exhaust flow restricting devices have been found less satisfactory. The manually adjustable type of valve is disadvantageous in that the restriction area set before starting the tool and causes an outlet flow restriction and a motor speed limitation not only at idle running or at low-load operation, but causes a power output limitation also at lower speed levels, i.e. during the final tightening of a screw joint. This means that the full capacity of the power wrench is not available during the final tightening stage.
The self-adjusting type of restriction valve on the other hand has turned out to be very difficult to make work properly at different air pressure and outlet flow levels. Accordingly, this type of valve is very sensitive to changing exhaust flow magnitudes and has a tendency to restrict the flow too much or too little. The intended automatic continuous adjustability of this type of valve is very difficult to obtain.